β”-(CNB-EDT-TTF)4BF4; Anion Disorder Effects in Bilayer Molecular Metals

Sandra Rabaça,Isabel Santos,Manuel Almeida,Sandrina Oliveira,Elsa Lopes,Vasco Gama,Enric Canadell,Gonçalo Oliveira

doi:10.3390/cryst8040142

Sandra Rabaça, Isabel Santos + Show 6 more

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https://doi.org/10.3390/cryst8040142

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Abstract

The preparation and characterization of new salts based on the dissymmetrical TTF derivative CNB-EDT-TTF (cyanobenzene-ethylenedithio-tetrathiafulvalene) and BF4− anions, are reported. Depending on the electrocrystallization conditions salts with different stoichiometries, (CNB-EDT-TTF)BF4 and β”-(CNB-EDT-TTF)4BF4, can be obtained. The 1:1 salt is an electrical insulator isostructural to the ClO4 analogue previously described. The 4:1 salt is a new member of the family of 2D metals of this donor with different small anions X, (CNB-EDT-TTF)4X, characterized by a bilayer arrangement of the donors and it was obtained in a monoclinic polymorph with a β”-type donor packing pattern. The small anions in this compound are severely disordered between the donor bilayers, which present slightly larger lattice parameters than the isostructural ClO4 analogue. Both electrical conductivity and thermoelectric power measurements in single crystals denote metallic properties as predicted by electronic band structure calculations. As a consequence of the anion disorder the metallic regime of the electrical conductivity denotes electronic localization effects with a progressive increase of resistivity below ~25 K. Because of the larger lattice parameters the intermolecular interactions and electronic bandwidth are decreased compared to other (CNB-EDT-TTF)4X salts. The large and positive thermoelectric power S of this compound (~110 μV/K in the range 100–330 K) and its electrical conductivity σ = 20 S/cm at room temperature lead to a power factor S2σ = 24 μW/K2m, quite large among molecular conductors, placing these compounds as potential candidates for thermoelectric materials.

Highlights

It has been recently shown that the dissymmetrical TTF derivative cyanobenzene-ethylenedithiotetrathiafulvalene (CNB-EDT-TTF) (Scheme 1) is an electron donating molecule that when combined with small anions X such as ClO4 −, PF6 −, and I3 −, leads to a family of radical cation salts of composition (CNB-EDT-TTF)4 X and characterized by a unique donor bilayer structure [1]
As previously observed with the anion ClO4 − depending on the solvent and electrocrystallization conditions salts with 1:1 and 4:1 donor:acceptor stoichiometries; (CNB-EDT-TTF)BF4 (1) and (CNB-EDT-TTF)4 BF4 (2) can be obtained, the first one as black block shaped crystals from tetrahydrofuran (THF) and the last one as thin dark brown platelet shaped crystals from dichloromethane solutions
Its structure and properties are comparable to the other compounds previously described, but the anions are more disordered and in spite of the smaller anion size the lattice parameters are slightly larger and intermolecular interactions smaller, while thermoelectric power becomes larger

Summary

Introduction

It has been recently shown that the dissymmetrical TTF derivative cyanobenzene-ethylenedithiotetrathiafulvalene (CNB-EDT-TTF) (Scheme 1) is an electron donating molecule that when combined with small anions X such as ClO4 − , PF6 − , and I3 − , leads to a family of radical cation salts of composition (CNB-EDT-TTF) X and characterized by a unique donor bilayer structure [1] This bilayer arrangement is promoted by an effective arrangement of hydrogen bond dimeric C–N···H–C interactions involving the cyano groups, as a combination of R2 2 (10) and R2 2 (8) synthons in a two-dimensional network.

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